4 Groundwater quality status and trends in relation to controlling factors
The general controls on groundwater quality and its evolution over time are discussed above in section 1.3. In this section the aim is to identify and explain significant relationships between groundwater quality in New Zealand (status and trends) and factors of potential influence, such as well depth and aquifer characteristics (cf. Aller et al, 1987) or surrounding land use. Note that well depth and aquifer confinement must be assessed together because they are correlated: for the sites considered in this study, there is a statistically greater proportion of shallow wells (less than 10m deep) in unconfined than in confined aquifers (see section 1.2).
4.1 Well depth and aquifer confinement
The Kruskal-Wallis test does not reveal any significant relationships between trend magnitudes and well depth or aquifer confinement for any of the parameters considered in this study. There are, however, significant (p < 0.05) relationships between the median values of four groundwater quality parameters and well depth and/or aquifer confinement.
- Median NO3-N concentrations are higher for shallow wells in unconfined aquifers (Figure 10).
- Microbiological indicator parameters are also higher in shallow wells in unconfined aquifers (Figure 11).
- Median SiO2 concentrations are lower for shallow wells in unconfined aquifers (data not shown).
- Median PO4-P concentrations are lower for shallow wells in unconfined aquifers (data not shown).
The first two relationships almost certainly arise from human influence. On the one hand, these relationships are to be expected, because unconfined aquifers are more susceptible to contamination, and proximity to the surface (the source of NO3-N and microbial pathogens) is obviously an important influence. However, it is important to stress that these relationships are not even remotely predictive, because it is clear from Figures 10 and 11 that deep wells in confined and semi-confined aquifers can also be susceptible to contamination by NO3-N and/or micro-organisms (which is probably why there is no significant relationship between the rates of change in these parameters and well depth or aquifer confinement).
Cases of microbial contamination in deep wells or confined aquifers probably reflect poor well-head protection more than the susceptibility of a particular type of aquifer (Sinton, 2001). Cases of NO3-N contamination in deep wells or confined aquifers may also indicate poor well-head protection, but can arise in certain New Zealand aquifer systems where the groundwater tends to remain oxidised for a long distance along the flow path (eg, Canterbury), possibly due to a low concentrations of the organic matter required as the substrate for microbial denitrification (Langmuir, 1997).
The third and fourth relationships probably reflect natural processes of water-rock interaction: SiO2 and PO4-P accumulate naturally in groundwaters due to the dissolution of minerals (Langmuir, 1997). The relationship might be biased to some degree by the fact that many of the wells in acidic volcanic aquifers considered in this investigation happen to be quite deep, and groundwaters in such lithologies typically have SiO2 and PO4-P concentrations two to three times higher than other rock types (section 4.2).
Figure 11: Relationship between the microbiological parameter with the highest median (MaxMicro) and bore depth and aquifer confinement
4.2 Aquifer lithology
There are no significant relationships between trend magnitudes and aquifer lithology for any of the parameters considered in this study, but there are several significant relationships between aquifer lithology and the median values of certain water-quality parameters. Many of these relationships are most evident for deep or confined aquifers, where water-rock interaction has caused the chemistry of groundwater to be influenced by the chemistry (mineralogy) of the aquifer.
For example, in accordance with expected mineralogy, SiO2 concentrations are lowest in carbonate aquifers and highest in aquifers of volcanic character, especially ignimbrite and rhyolite. There are several other relationships like this, many of which have been reported previously (Daughney and Reeves, 2005). Aquifer lithology also has an indirect influence on the concentrations of redox-sensitive elements and compounds such as NO3-N, NH4-N, Fe, Mn, As and SO4. The transition from the oxidised to reduced state is mediated by microbial respiration (see section 1.3), which requires the presence of a reductant (organic carbon in most cases). In aquifer lithologies with low concentrations of organic carbon, such as ignimbrite, rhyolite and some gravels (eg, Canterbury), it is possible that microbial respiration is curtailed. As a result the groundwater remains oxidised, and this favours the persistence of NO3-N and SO4 and prevents the solubilisation and accumulation of NH4-N, Fe, Mn and As (Langmuir, 1997). The opposite case applies to lithologies that do contain abundant organic carbon, which is why concentrations of dissolved Fe and Mn are often observed to be high in sand aquifers (Daughney, 2003).
4.3 Surrounding land use and land cover
Elevated levels of nitrates and faecal bacteria in groundwater are almost certainly the result of human activities on the land. This conclusion is supported by the fact that elevated nitrate and bacteria levels are more often observed in shallow, unconfined aquifers (ie, those that tend to lie under flat expanses of urban and rural land) than deep, confined aquifers; see section 6.1. Furthermore, numerous catchment or local-scale studies have suggested land uses such as agriculture and horticulture as major sources of nitrate contamination of groundwater (eg, Close et al, 2001) and this helps develop our understanding of land use impacts on groundwater quality at a national scale.
While land use is logically implicated in the degradation of groundwater, this study has not been able to reveal systematic relationships (at a national scale) between the particular type and intensity of land use surrounding monitoring bores and the magnitude or pattern of impact from these land uses on groundwater quality. This result applies to all groundwater quality parameters reported earlier, including NO3-N, SO4, conductivity (and other potential indicators of land use impact), and holds regardless of whether all monitoring sites are considered together, or if the data set is limited to the sites less than 10m deep (at which the impact of land use would probably be most apparent).
The lack of a proven relationship between land use types/intensity and groundwater quality is a common result that has been observed in several previous studies in New Zealand and overseas (Close et al, 1995; Reijnders et al, 1998; Broers and van der Grift, 2004; Daughney and Reeves, 2005, 2006). There are a number of reasons for this:
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an overwhelming majority of groundwater quality monitoring wells are located in developed catchments and it is therefore hard to make robust comparisons with groundwater in undeveloped areas (ie, pristine groundwater)
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the quality of groundwater drawn from a monitoring well does not necessarily reflect the land use directly surrounding that well; water may have travelled from an area dominated by a different type/intensity of land use
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groundwater that has been impacted by land use might not have had sufficient time to move all the way from the aquifer’s recharge area to the monitoring well, or substances indicative of land use impact (eg, nitrate) might have been transformed or degraded before reaching the monitoring site.
In order to better understand relationships between groundwater quality and different types of land use, accurate knowledge of the age and source of the groundwater being monitored is required. As stated in Section 2.3, such information is not generally available for monitoring wells in New Zealand, and to obtain this information will require substantial research effort in the future.
